Tapered aperture horn antenna for electromagnetic energy below 40 megacycles



July 11:, 19.61

B. G. HAGAMAN 2 992,429 TAPEREID APERTURE HoRN ANTE s NNA. FOR ELECTROMAGNETIC ENERGY, BELOW 40 lMEIGCYCLES Eled Feb. 17', 1959 Church, Va., assignor to Inc., Washington, D.C., a corpora- The present invention relates to an antenna of the electromagnetic horn-type for radiating radio waves directionally lat a desired low angle to the earths surface over a broad frequency band.

It is often desirable to transmit radio waves at different frequencies in the high frequency band of about 3 to 30 megacycles. It is also desirable in communicating over long distances at these frequencies to maintain the take-off angle as low to the earth as possible, for example, two to three degrees. Applicant has found it desirable even at the long wavelengths of these frequencies to use a very large horn antenna in order to obtain a very desirable directional radio beam. Using horizontally polarized waves, the height to the top of the horn aperture must be about three wavelengths above the earth in order to provide the desired low take-olf angles. At about seven megacycles, ywhere the wavelength is approximately 150 feet, it is possible to obtain the desired low take-0E angles by employing a horn aperture which is about 450 feet high. However, at frequencies near the low portion of the high frequency range, for example, at 4 megacycles one wavelength equals 25() feet and to obtain the desired low angle take oif requires an antenna size which is very expensive or impractical to build. It is an object of the present invention to overcome this difficulty using only a single horn antenna capable of radiating a highly directional radio beam at the desired low take-off angle over a wide frequency range.

According to the invention radio waves in a range of approximately 4 to 24 megacycles are radiated at takeoif angles of two to three degrees by means of a single horn antenna capable of radiating both horizontally polarized waves and vertically polarized waves, the antenna being energized with vertically polarized waves in the lower portion of the frequency band and wth horizontally polarized waves in the higher portions of the frequency band. For the vertically polarized waves the take-off angle is not dependent on the height of the aperture, but is dependent on the earths constants, which may be controlled to a considerable extent by the use, for example, of ground wires. At the higher frequencies of the frequency band starting, say, at 7 megacycles the horn height is such that for horizontal polarization the take-off angle will be adequately low. Therefore the horn provides means for obtaining vertically polarize ,iation over the lower portion of the frequencyv ran Y for example, 4 to 7 megacycles, and horizontal polariz tion over the range of say 7 to 24 megaoycles. In this manner the full range of 4 to 24 megacycles 'can be advantageously covered using a singleantenna having the desiredlow takeoff angle throughout the entire frequency range.

Another object or feature of the invention is a feed system for the antenna which raises the effective center of radiation of the antenna to a considerable height and thereby aids in obtaining a low take-off angle for the horizontally polarized waves.

Another object or feature of the invention is a multiple feed system which permits several transmitters to be coupled to the single antenna and operated independently.

Other objects and advantages of the invention will be fully understood from the yfollowing description and the drawings in which:

hatented Juiy il, 1961 The figure schematically shows one embodiment of the invention.

Referring to the drawings there is shown an antenna comprising a radiating horn 10 and a pair of vertical wave guides 11A and 11B. Although horn antennas and wave guides are generally used for microwaves only, the present antenna system is designed for radiating waves having wavelengths of the order of feet or more. For the purpose of illustration it will be l.assumed that the antenna is being used for transmitting radio waves although it will be understood that it could also be used for reception. Radio translating equipment 12, which may be a high frequency transmitter capable of producing radio waves at various frequencies which, for the sake of definiteness will be assumed to be in the range of 4 to 24 megacycles, is connected by transmission lines 14 and 15 to a pair of radiators 16 and 17. Radiators 16 and 17 may be horizontal probes extending from opposite walls of the wave guides toward a central partition 18 which vertically bisects wave guides 11A and 11B. I-t will be understood that the partition 18 as Well as the walls of the horn antenna and wave guides 11A and 11B need not be formed of a solid metal but may be, for example, wire mesh or wire spaced close enough together to simulate a solid wall. The wave guide 11 may have a height of several hundred feet and accordingly will rquire a suitable structure such as a tower for supporting the same. The horizontal radiators or probes 16 and 17 enter the wave guides 11A and 11B near the bottom. The direction of radiation from the wave guide is vertical and thus avoids the necessity of raising the extremely large and heavy feed elements into the air several hundred feet. At the top of wave guide 11 there is a reflecting wall 20 which may be at an angle of 45 degrees to the horizontal. The wall 20 forms the top of the uppermost section 19 of wave guide 11. The partition 18 terminates near the bottom of the top section 19. The horn 10 preferably has eight tri-angular sides arranged as shown to form a square or diamond-shaped aperture 21 with one diagonal horizontal and the other diagonal vertical. The walls of horn 10 are joined to the horizontal and vertical sides 22-25 of the top section 19 as shown. In one wall of horn 10 near the edge 25 a vertical radiating probe 28 is mounted in the horn. Probe 28 is connected to the radio translating equipment or transmitter 12 by a suitable transmission line 29. Isolation between the horizontally polarized field produced by the horizontal radiators i6 and 17 and the vertically polarized iield produced by the vertical radiator 28 is obtained by virtue of the differences in the polarization and location of the radiators. Isolation between the horizontal radiators is obtained by virtue of the isolation of the wave guide sections. If desired further isolation between the vertically and horizontally polarized fields can be obtained by means of a grid 30 consisting of vertical wires suspended immediately Iahead of the inclined Wall 20.

Let it be assumed that the system is required to radiate Waves which have a given frequency which may have to be varied from time t0 time in the range of 4 to 24 megacycles. Under these conditions, waves having a frequency of say 7 to 24 megacycles are supplied simultaneously or individually to transmission lines 14 and 15. Therefore, in this frequency range, horizontally polarized waves produced -by radiating probes 16 and 17 are sent through the wave guides 11A and 11B to reflecting surface 20 and then transmitted as horizontally polarized waves by the 'horn 10. Either or both transmitters may be employed. When a transmitter is operating at a frequency between 4 and 7 megacycles the output of the transmitter only is supplied to transmission line Z9 so that the horn is excited by vertically polarized waves produced by the radiator 28. In this frequency range, the ham 10 is capable of radiating waves at a very small take-off angle such as 2 to 3 degrees. At four megacycles the wavelength is approximately 250 feet and if the height of the horn were, say 450 feet the height would be less than 2 wavelengths above the earth. Nevertheless it is found that even at such low heights it is possible to maintain very low takeoff angles when vertical polarization is used. It is found that the take-off angle over the frequency range of 4 to 7 megacycles will not be dependent ou the height of the antenna. At 7 megacycles and above for, the dimensions given herein, the take-olf angle of a horn will be adequately low for horizontal polarization, thus the desired low take-olf angle for radio transmission is obtained -at any operating frequency in the range of 4-24 megacycles. It will be understood of course that the particular dimensions and frequencies mentioned herein are given for the purpose of definitely describing one embodiment of the invention and are to be considered merely illustrative and that other dimensions and frequencies may be used. Indeed many modifications and variations of the principles of my invention will be apparent to those skilled in the art and accordingly the invention is not to be construed as being `limited except as defined in the following claims.

I claim:

l. An antenna system for radiating radio waves of two widely spaced lfrequencies in the high frequency range directionally at a very low angle of elevation comprising an electromagnetic horn mounted within a few wavelengths of the earth and having a longitudinal axis at a very small angle to the earths surface and having an aperture height of substantially several hundred feet; means for maintaining the angle of elevation of said waves relative to the earths surface at said very low angle, said last means comprising, radiating means for `exciting said horn with vertically polarized waves, radiating means for exciting said horn with horizontally polarized waves, means for generating radio waves vat a frequency in a low frequency portion of said range and supplying them to the vertical polarization radiating means and supplying waves having a frequency in the high portion of said range to the horizontal polarization radiating means.

2. An antenna according to claim 1, wherein said horn has a diamond shaped aperture having a horizontal and a vertical diagonal.

3. An antenna system according to claim l, wherein a wave guide extends upwardly and is joined to the feed end of the horn, said radiating means for the horizontally polarized waves being mounted in said wave guide and near the bottom thereof.

4. An antenna according to claim 3, wherein said horizontal polarization radiating means comprises a pair of horizontal probes extending into said wave guide from opposite side Wall thereof.

5. An antenna according to claim 4, wherein said wave guide includes a metallic vertical partition parallel to said opposite side Walls.

6. An antenna system for radiating radio waves directionally atan angle of elevation of a few degrees comprising a ared electromagnetic horn having a longitudinal axis at a very small angle to the earths surface and located within a few wavelengths thereof and having a quadrangular `aperture having substantially vertical and horizontal diagonals, means for exciting said horn with vertically polarized waves, means for exciting said horn with horizontally polarized waves, means for generating said radio waves `at an adjustable frequency within a frequency range of 3 to 30 megacycles and means for supplying said waves having a frequency in the low portion of said range to the vertical polarization radiating means and supplying waves having a frequency in the high portion of said range to the horizontal polarization radiating means.

7. An antenna system yfor radiating radio waves directionally at a very low angle of elevation comprising an electromagnetic horn mounted substantially at earths surface and dimensioned to radiate waves having frequencies in a range below 40 megacycles and having an aperture having a height of substantially several hundred feet, means for exciting said horn with vertically polarized Waves, means vfor exciting said horn with horizontally polarized waves, means for generating said radio waves at various frequencies within said frequency range and means for supplying said waves having a frequency in the low portion of said frequency range to the vertical polarization radiating means -and supplying waves having a frequency in the high portion of said frequency range to the horizontal polarization radiating means.

8. An antenna for radiating or receiving radio waves directionally comprising a iiared electromagnetic horn having a quadrilateral aperture, said horn having eight triangular sides each extending from the aperture to the small end of the horn, the bases of four yof the triangular sides being the edges of the aperture and the bases of the other four triangular sides being the edges of the small end of the horn.

9. An antenna for radiating or receiving radio waves in the high frequency range of 3-30 megacycles directionally at a very low angle to the earths surface, comprising a flared electromagnetic horn having a longitudinal axis of a very small angle to the earths surface and having a diamond-shaped aperture with substantially vertical yand horizontal diagonals, the horn having eight triangular sides each extending from the aperture to the small end of the horn, the bases of four of the triangular sides being the edges of the aperture and the bases of the other four triangular sides being the edges of the small end of the horn.

l0. An antenna according to claim 9, wherein the aperture has a vertical extent of the order of a wavelength at the lowest frequency of said frequency range and the small end of said horn is at a comparable height `above the earth, a vertically extending electromagnetic wave guide, said wave guide extending substantially from the surface of the earth to the top of the small end of the horn, the upper end of said wave guide being connected to said horn and communicating with the small end thereof.

1l. A radio antenna comprising a flared electromagnetic horn dimensioned for receiving or transmitting horizontally -and vertically polarized electromagnetic waves having frequencies in the frequency range of 3-30 megacycles, said horn having a longitudinal -axis at a very small angle to the earths surface, said horn having its sides inclined so that the horn aperture has a width which tapers from the center to the ends of the aperture in both the horizontal and vertical directions, said horn having -a plurality of triangular sides each extending from the aperture to the small end of the horn, the bases of half of the triangular sides being the edges of the aperture and the bases of the other half of the triangular sides being the edges of the small end of the horn.

l2. An antenna according to claim 1l, wherein said aperture is quadrilateral -and said horn has eight triangular sides.

References Cited in the file of this patent UNITED STATES PATENTS 2,362,561 Katzin Nov. 14, 1944 2,719,230 Smoll et al. Sept. 27, 1955 2,851,686 Hagaman Sept. 9, 1958 OTHER REFERENCES Pub. I, Dual-Mode Horn Feed for Microwave Multiplexing, by Levine et al., Electronics, September 1954, pp. 162-164. 

